Many refrigerators have an automatic ice making system built inside. After ice cubes have been produced by an ice maker, they are collected in an ice trough, and an ice level detection means is provided to detect whether the ice level of the accumulated ice cubes has reached a high limit beyond the storage capacity of the ice trough and determine whether to stop ice making. If the ice level detection means malfunctions the ice maker will continuously make ice and the ice cubes will accumulate and result in damage of ice sweeping element and ice making tray or the like. Refer to FIG. 1 for the general structure of a conventional ice maker. It includes a control box to provide an ice making process, an ice making tray, an ice sweeping element and an ice trough located at a lower side of the ice maker (may be a retrievable ice collection tray as shown in the drawing). There is an ice level detection means located above the ice trough. The conventional ice level detection means has a linear detection device. Take the ice making process of a conventional ice maker as an example. After ice cubes have been produced in the ice making tray, the control box activates the ice sweeping element to move the ice cubes from the ice making tray to the ice trough to be accumulated. The control box temporarily suspends ice making process in the ice making tray during the ice sweeping operation. If the ice cubes accumulated in the ice trough have reached a saturated condition and arrived a lower limit detection position of the linear detection device, the linear detection device is pushed by the accumulated ice cubes and the ice sweeping element stops ice sweeping action. Namely the ice maker remains at the ice sweeping step and suspends the ice making process. The linear detection device performs detection through a horizontal line. As cracks are often formed among the ice cubes, the linear detection device could fall in the cracks without being pushed. Hence during the next ice sweeping operation the ice cubes accumulate continuously and squeeze the linear detection device that could cause malfunction of the linear detection device. As a result, the ice maker continuously produces ice and the ice cubes are accumulated in the ice trough and ice making tray. After a period of time, the ice cubes thaw and bond together to become a big and hard ice chunk. The ice sweeping element, ice making tray and control box could be damaged. In such a condition even if users have found out the problem they cannot immediately clear the ice trough. As the control switch of the ice making system usually is located outside the control box, users have to cut off electric supply of the entire refrigerator to melt the ice cubes in the ice trough before clearing the ice chunk. It often happens that the linear detection device is deformed and damaged beyond repairs by the pushing stress of the ice cubes. Hence to provide improvement for the ice level detection means of the ice maker is an issue remained to be resolved in the industry.